Detroit just took another huge step towards the abyss, with its proposal to turn off street lights across half the city. This is the nightmare scenario for anybody who loves a particular city: that one day, it'll fail. Chances are, if you live in (or near) a city, you already worry over every little sign that your town is getting less cool, less vibrant, or just crappier.

Cities grow, or they die. We've all seen the spectre of urban centers hollowing out from the inside. And "ruin porn" has become a whole category of photography, with a huge fanbase. There have been multiple books of photos of Detroit's dilapidated theaters, railway stations and other formerly grand buildings. There's just something insanely compelling about looking into a formerly vibrant city gone dead — and part of it is the fear that this could happen to your town, as well.

So here are some tips on how to tell if your city is next in line to be someone else's ruin porn:

The size of the population is actually going down.
This is the big one, says Patrick Condon with University of British Columbia. "Sadly, many center cities are seeing that." Many cities have lost 50 to 70 percent of their peak population — like Detroit, whose population has dropped 60 percent. "St. Louis has the worst population loss of any large city," adds Brent Ryan, an Assistant Professor of Urban Design and Public Policy at MIT. Other cities with huge population loss: Cleveland and Baltimore, and other big cities.

Adds Ryan:

If you keep losing people, eventually you're going to lose houses, and then you end up with long-term vacant housing. Initially, you lose people and you just un-crowd, so you end up with fewer people in a house. And you say, "Cool, I can have this house all to myself." But eventually there aren't enough people even at that level to fill up all the houses, and you end up with permanently vacant houses. There's actually a whole typology of how long has the house been vacant: Does it have trees growing out of the roof? Is it collapsing in on itself? Is the wall falling in? How many fires have there been?

The average income of the residents is going down.
In other words, "people moving in have lower incomes than those moving out," says Tom Bier, Senior Fellow with the Maxine Goodman Levin College of Urban Affairs at Cleveland State University.

The population is getting older.
And as the city's population gets older, smaller and poorer, the reverse is happening in the suburbs, says Condon: younger families, growing populations and increasing average income. Adds Condon, "Detroit is the classic nighmare where the center city was vacuum cleaned of its middle class by a GM-inspired highway building boom, devaluing center city lands and sucking all the life into far flung isolated cul de sacs."

People aren't paying taxes or mortgages
Bier says there are a number of classic signs that a city is in trouble:

An increase in property tax delinquency.

An increase in mortgage delinquency and foreclosure.

An increase in rental of single-family homes.

Property owners are unable to make needed repairs, because they don't have enough money.

Homes are being sold through "rent to own."

There's an increase in property code violations — or if the city isn't doing inspections, then that's a huge red flag.

Says Bier, "The key to preventing decline is making — or keeping — the community attractive to the point where people who can readily afford to maintain property will choose to live there. Once real estate begins to show lack of maintenance, stronger incomes will go elsewhere, which results in more decline, which pushes more stronger incomes away — and on and on."
A "signature" building is standing vacant for decades
Americans especially hate to see local "monumental buildings" standing vacant, because it makes their towns look bad, says Ryan, who's the author of Design After Decline: How America rebuilds shrinking cities
. Detroit has had several "spectacularly vacant" buildings for decades, which become "icons of decline," says Ryan. These include the Packard Factory, the main local hotel, the train station — and even the baseball stadium for a while. When you see these huge vacant buildings around town, "that's sort of the clearest indication."

You can't find any use for the land at all.
In most cities, a parking lot is viewed as the absence of buildings — but in Detroit, even a parking lot is a welcome sign that the land is being used for something, says Ryan. If you can't even build a parking lot on some vacant land, then it's a sign that there's absolutely no activity going on. "At least someone wants to use the land for something. They're using it to park a car. It's not nothing."

A city is growing, but there are huge dead areas
On the surface, says Ryan, Philadelphia appears to be doing quite well — its population has been growing in recent years, and it has an image of being a vibrant, exciting city. But you can't take a train through Philadelphia, you'll go through North Philadelphia, "the heart of dead industrial Philadelphia." Philly was once the greatest industrial city in the United States — and that's all gone now. "The 30 percent of the city that was not industrial... is very active and vibrant. But you've got huge dead areas of the city," says Ryan. The healthy areas of the city are masking the trouble spots.

The complexities of urban decay

The actual death of a city is much more unlikely than you might think — even though cities can decline and fall apart, that doesn't necessarily mean they're dead. Says Robert A. Beauregard, Professor of Urban Planning at Columbia University, "Try not to think that the city is like a human being who is born, lives, and dies. Cities are socio-technical systems, precariously integral, and capable of becoming smaller and fragmented and still functioning well." When cities do disappear outright, they're usually smaller and more temporary, like the Gold Rush towns in California in the 19th Century.

And cities do come back — in the early 1990s, Los Angeles was going to Hell, even before the L.A. riots. San Francisco's population dropped steadily, losing about 150,000 people until about thirty years ago. And then SF started gaining population again — and now SF's population is the biggest it's ever been, says Ryan. Meanwhile, Chicago started gaining population again after a long period of loss, "and now it's started losing them again. Maybe Chicago's actually dying, and we can't tell."

Ryan says he first got interested in this topic when he visited Detroit in 1993. His friend, a local reporter, told him to check out the local train station before he left town. "Basically, the train station was like a Grand Central Station in New York, except it was abandoned and open. And it basically remains abandoned to this day," says Ryan. "All of a sudden, we were in this 'neutron bomb' sort of atmosphere, where you walk down the streets and nobody's there. We went in the doors and all of a sudden it was like we were in the Baths of Caracalla, except it was after the apocalypse. It was like Rome in 419 A.D."

There were signs of recent fires, debris everywhere, and signs that people had recently been there. Standing inside that train station was a heady mixture of "total menace" and "overwhelming sadness" that this magnificent was in ruins. This was the "emotional catch" that's drawn Ryan into studying dying cities, and that's the reason why so many of us are fascinated with looking at ruin porn.

When Ryan started writing about these topics a decade ago, everybody kept insisting that Detroit and other cities were mounting a comeback — but it was obvious if you looked at the facts, that it wasn't true. If you look back at newspapers, every year since 1960 there's been at least one article saying that Detroit is finally on an upswing. The crash of 2007 and the ensuing economic disasters have highlighted the already-existing troubles of America's cities, and that's when things like "ruin porn" have become so popular. "It's very visceral," says Ryan. "It really gets you."

And actually, ruin porn is deceptive — in a lot of neighborhoods of dying cities, the wilderness has already reclaimed the city, because the wooden houses have collapsed completely and greenery has taken over. It's actually green and tranquil, not gothic and horrifying — and in Detroit, there are beavers now. "These are cities that were built fast and cheap and built by speculators, to make quick money," says Ryan. "It turns out the whole fabric of the American city is quite ephemeral."

Sometimes it's just not clear if an area is suffering a temporary downturn — maybe the overbuilt suburbs of Vegas and Phoenix are doomed to fall apart, or maybe they'll be reabsorbed when the economy picks up again, says Ryan. Or maybe they'll be waiting and waiting for an uptick that never comes. "It just becomes a race to demolish, and that's a very late stage of this process," says Ryan.

And the American mentality is always to move on and find a better place if your city is in trouble. It's the "Go West, Young Man" mindset, says Ryan. "A lot of people say that. They say, 'God, Detroit? New Jersey? I'm never going back there. It's very different from Europe, where cities are the heart of the nation's culture. If Paris dies, then France is dead. The notion of being French is dead. And since that's impossible, it's impossible for Paris to die. Whereas in America, a city is like a business. And if you go out of business, then well, you just weren't doing a very good job. Too bad for you. Better luck next time."